New research published in the Journal of Experimental Botany shows that wheat crops are more vulnerable to heat wave damage when these events occur before the plant reaches its flowering stage — a critical time when grains begin to form — compared to heat waves that happen later in the growing season. However, the study also found that if wheat is exposed to a heat wave early in the season, it may be "primed" to withstand later heat waves, reducing the damage they cause. This protective effect suggests that early stress can help plants prepare for future challenges. A team of researchers from the University of Lleida in Catalonia, Spain, studied how heat wave intensity, frequency, and timing — especially in relation to the flowering stage — affect wheat yield. As heat waves become more frequent and severe globally, they are increasingly overlapping with key periods in the wheat growing season, including flowering, when the plant transitions from forming grains to growing them. Understanding how these factors interact is essential for predicting how climate change will affect food production. To test their findings, the researchers conducted field experiments over two growing seasons using two modern wheat varieties. Despite their different strategies for building yield, both varieties responded similarly to heat wave treatments. These treatments were created by growing wheat in transparent tents that mimicked heat waves through a greenhouse effect, raising temperatures above normal levels. The researchers standardized conditions to compare how pre-flowering and post-flowering heat waves affected the crops. They measured heat load — the accumulated temperature above normal — with a focus on temperatures over 32°C, as this is when heat stress becomes particularly damaging. The study found that wheat is more sensitive to heat waves before flowering, which can reduce the number of grains produced, while heat waves after flowering mainly affect the weight of the grains. Surprisingly, wheat that experienced an early heat wave showed a "priming" effect, meaning it was better able to withstand a later heat wave. This is similar to how some organisms build up a kind of memory or resilience after experiencing stress, allowing them to respond more effectively to future challenges. These findings can help farmers improve crop yield predictions and target adaptation strategies to the most vulnerable stages of the growing season. The research underscores the need to protect wheat during the flowering stage, when the number of grains is determined. While the findings are specific to wheat, they may also apply to other temperate crops like barley, oats, and rye, though each species will have different heat thresholds. As global temperatures rise, understanding how multiple heat events affect crops will be crucial for maintaining food security. Wheat, a staple crop for billions of people, is especially vulnerable to heat compared to tropical crops like maize or rice, making it a priority for climate adaptation efforts.